With normal vision, an individual is able to focus at objects located at different distances. Ideally, an individual is able to focus on distant objects, referred to as distance-vision, and on near objects, referred to as near-vision. The optical system of the eye uses numerous muscles to change the focus between these distances. These muscles adjust various aspects of the eye when transitioning between distance-vision and near-vision. The muscle adjustments include making subtle changes to the shape of the crystalline lens to adjust the focus of the lens, rotating the eyeballs to rotate their optical axes, and changing the size of the pupils.
Presbyopia is a natural deterioration of near vision, caused by loss of flexibility in the eye's crystalline lenses as one ages. Presbyopia can be partially compensated by wearing “reading” glasses that correct near-vision refraction errors, so that the eye does not have to focus as strongly when gazing at near objects. Presbyopic persons need different optical corrections for near-vision and for distance-vision. However, using two eyeglasses and changing them frequently is distracting. To avoid continually exchanging eyeglasses, bifocals may be used that offer different optical corrections for near-vision and for distance-vision. The transition between these two vision regions can be abrupt or, gradual. The latter eyeglasses are called Progressive Addition Lenses (PALs). Abrupt change bifocals have a visible line separating the two vision regions, while PALS have no lines or edges visible between the regions with different dioptric powers.
In spite of all this progress, some types of vision-related discomforts still persist. One of these discomforts is related to a shift of habits in the modern, digital lifestyle. A large and increasing fraction of professions require workers to spend a large and increasing fraction of their working time focusing at close-distance digital interfaces, including computer screens and mobile devices. The same is true for the private lives of many, spending hours playing video games, texting and checking updates on cell phones, among others. All these professional and behavioral shifts rapidly increased the time people spend looking at digital screens, devices, displays, and monitors at a much closer distance than before. The increased time of the eye being trained at near-vision images places excessive demands on the muscles involved in near-vision, often straining them beyond the comfort zone. This can lead to fatigue, discomfort, pain, or even digitally induced migraines. Up to now, there is no widely accepted consensus on the precise causation mechanism of these digital-device related visual discomforts, pains and migraines, even though millions of patients experience these pains every day. Therefore, there is a need for glasses, or other optometric solutions than can provide relief for digital eye discomforts.
FIGS. 1-4 illustrate the basic problem of binocular misalignment. FIG. 1A illustrates that when we look at a near object, like the shown cross, our vision accommodates in two ways. First, we accommodate the optical power of our eyes 1-1 and 1-2 to image the near object at a distance L onto the retina of each eyes. This is often called the accommodative response A. Second, we rotate our eyes 1-1 and 1-2 inward by an angle α, so that the visual axes 2-1 and 2-2 of the eyes are pointing at the same near object. This response is often called the accommodative convergence AC. For obvious geometric reasons, the angle α of the accommodative convergence AC, relative to the straight forward reference axis, is directly related to the distance L of the accommodative response A: α=α(L). For healthy, well-aligned eyes the ratio of the accommodative convergence AC to the accommodative response A, AC/A, is a geometrically well-defined function, depending on the object distance L and the pupil distance PD of the two eyes.
FIGS. 1B-C illustrate that eyes often display various forms of accommodative misalignments. In FIG. 1B, the two eyes each turn inward, but to a lesser degree that geometry would require. This leads to the accommodative convergence angle α being less than geometrically necessary by a misalignment angle β. In some detail, the visual axes of the eyes 2-1 and 2-2 should point into the direction denoted as the necessary accommodative alignment to properly see the near object, but, instead, they turn inward to a lesser degree and instead point to the direction denoted as relaxed or natural accommodative alignment.
FIG. 1C illustrates a case, when this lesser turn is asymmetrical. In the shown case, the visual axis 2-1 of the first eye 1-1 properly points to the direction of the necessary accommodative alignment, while the visual axis 2-2 of the second eye 1-2 is turned inward only to the direction of the relaxed or natural accommodative alignment, that is misaligned by the accommodative misalignment angle β.
FIGS. 2A-D illustrate some types of accommodative misalignments. The definitions of misalignments used by different schools of optometry and by monographies show some discrepancies, and the techniques to characterize these misalignments are also varied. Therefore, the here-shown definitions are meant to be illustrative only, and analogues and equivalents are also within the scope of the illustrated terms.
To place the discussed misalignments into proper context, first the concept of fusing images is introduced. When our two eyes look at the same object, each eye creates its own visual perception. These perceptions are relayed from the eyes to the visual cortex, where the brain fuses the two images and creates a three dimensional (3D) perception of the viewed object. With optometric diagnostic systems, it is possible to test this image fusing. For example, two separate objects of the same shape can be separately projected into the two eyes with deflections, prisms, and mirrors that make the two projections appear to come from a single object. These visual perceptions will be fused by the brain into a perceived single image. Objects projected in this manner are called fusible objects, presenting fusible images.
If in an experiment the distance between the two objects is increased, or the deflection angles are increased, or the shapes of the objects are modified, then the projections into the two eyes start to differ. At some distance, or difference, between the objects, the discrepancy between the visual perceptions of the two eyes exceeds a threshold, and the brain stops fusing the two images into a single perception. Objects with such difference in distance, angle, or shape are called non-fusible objects, presenting non-fusible images.
With this preparation, FIGS. 2A-D illustrate the concept of fixation disparity, as measured by a test device, often called the Mallet box. The Mallet box displays two vertically aligned bars, and an “X O X” horizontal “anchor”. In some implementations, the two bars can be shifted sideways. In others, adjustable mirrors or prisms are placed in front of the patient's eye to achieve the same horizontal shift. With appropriate selective optics, the anchor and only one of the bars is shown for the first eye 1-1 as a centered bar 5-1-c, and the same anchor plus only the other bar is shown for the second eye 1-2 as a centered bar 5-2-c. The anchor and the centered bars 5-1-c and 5-2-c are clearly fusible. Accordingly, the brains of patients without accommodative misalignment problems will properly fuse these images.
FIG. 2B illustrates that patients with accommodative misalignments will not fuse the images properly. What is typically observed is that, while the images of the anchor, seen by both eyes, are properly fused into a single image, the bars are perceived as shifted. The first eye 1-1 perceives a shifted bar 5-1-s, while the second eye 1-2 perceives a shifted bar 5-2-s. The angle γ between the line to the image center and one of the visual axes 2-1 and 2-2 is called fixation disparity.
FIGS. 2C-D illustrate ways to measure the angle needed to counteract, or compensate the fixation disparity. In the system of FIG. 2C, the two bars are counter-shifted. A counter-shifted bar 5-1-x is shown for the first eye 1-1, and a counter-shifted bar 5-2-x is shown for the second eye 1-2. The bars are counter-shifted until the patient perceives the two bars as aligned. The angle corresponding to these counter-shifts, γ*, between the visual axes and line to the counter-shifted bars is measured and is typically referred to as an associated phoria. In the system of FIG. 2D, the bars are not counter-shifted. Instead, adjustable, or exchangeable prisms 7 are inserted in front of the patient's eyes. These prisms are adjusted or exchanged until the two bars are perceived as aligned by the patient. Then the prism angles, or the refraction angles of the refracted visual axes, are reported as the associated phoria γ*.
FIG. 3 illustrates how increasing a partial associated phoria partially compensates fixation disparity. Strictly speaking, the (full) associated phoria, that fully compensates fixation disparity, is given by the intersect of this curve with the partial associated phoria axis. If human vision were a purely optical process, the partial associated phoria would be simply equal to the negative of the partially compensated fixation disparity. Accordingly, the curve would be a straight line through the origin, tilted by −45 degrees, pointing from the upper left corner to the lower right corner. However, FIG. 3 illustrates that human vision is much more complex, and perception and image processing play crucial roles in it. FIG. 3 shows four types of relations between the partially compensated fixation disparity and the partial associated phoria. Visibly, none of these lines are straight, none of them go through the origin, and two of them don't even intercept the horizontal axis. These type II and III relations mean that no amount of partial associated phoria can compensate the fixation disparity in full. Therefore, it remains a substantial challenge to determine the associated phoria that fully compensates a patient's fixation disparity. A convention is mentioned in closing: the fixation disparity is referred to as “exo”, if the eyes do not turn inward to the necessary degree, while it is referred to as “eso” in those rare cases, when the eyes turn inward too much.
FIGS. 4A-C illustrate a related visual misalignment called disassociated phoria. To characterize disassociated phoria, an experiment similar to that in FIGS. 2A-D can be carried out, with the difference that instead of showing fusible images 5-1 and 5-2, the optometrists show non-fusible images 6-1-s and 6-2-s for the first eye 1-1 and the second eye 1-2. In FIG. 4A, these non-fusible images are the cross and the bar. As FIG. 4B illustrates, once the eyes are unable to fuse the images, often one or both of the visual axes rotate outward. In the shown asymmetric case, the visual axis 2-2 of the second eye 1-2 rotates outward by an accommodative misalignment angle δ. This angle δ of the outward rotation is measured and called disassociated phoria. In various applications, as below, the disassociated phoria is distributed over the two eyes evenly, thus the disassociated phoria per eye equaling δ/2. In some cases, e.g. as illustrated in FIG. 1C, the disassociated phoria δ may manifest itself unevenly and has to be distributed between the eyes accordingly.
FIG. 4C shows a particularly clear case, when simply no image is shown for the second eye 1-2, the view of the second eye 1-2 is blocked. This is an extreme case of non-fusible images. As for FIG. 4B, in response to the block, the visual axis 2-2 of the second eye 1-2 rotates outward by a measurable disassociated phoria angle δ.
As a quantitative characterization of accommodation misalignments, including fixation disparity and disassociated phoria, several practitioners use the misalignment-impacted AC/A ratio. The AC/A is a ratio of the accommodative convergence angle reduced by the fixation disparity, α−δ/2, (expressed with its tangent, in terms of “prism diopters” Δ), divided by the accommodative distance L, expressed in diopters D. A typical definition of AC is AC=100 tan(α−δ/2), in terms of prism diopters. For an average visual performance, an AC/A ratio of 6-6.5 Δ/D is necessary, while, remarkably, in large population segments the average of the misalignment-impacted AC/A ratio was measured to be about 3.5 Δ/D. Clearly, various forms of accommodative misalignment affect a large percentage of the population, and any progress towards relief from this is highly valuable.
A startling fact of the corresponding field of optometry is that the associated phoria angles and the disassociated phoria angles, determined by experienced practitioners, show remarkably wide variations. Experiments carried out on the same patient by different optometrists, and sometimes even by the same optometrist at different times, report phoria angles, expressed in prism diopters Δ, with a distribution having a standard deviation as much as 3Δ. (A prism diopter of 1Δ corresponds to a 1 cm prism refraction at 1 meter distance). The large variability of these methods precludes the effective determination and compensation of accommodative misalignments.
This exceptionally large standard deviation is probably due to several factors. These include the followings. (1) The methods of determination use the patient's subjective responses as key inputs. (2) Some methods use central images, while others use peripheral images for determining the associated phoria. The relative accuracy and relevance of these methods was not yet critically evaluated. (3) Most practitioners use a single measurement, or a single method, thus not benefiting from possibly important medical information that can be gleaned from carrying out multiple tests. (4) In a previous exploratory project, Applicants also discovered that the prismatic reaction of the eyes is quite different for moving test images. However, understanding the relation of optimal prismatic corrections based on static and moving test images is in its early stages. (5) While there are several ways to define prismatic misalignments, and they produce different prismatic predictions and diagnoses, eventually a single prism needs to be formed in the spectacles. It is far from obvious how to convert and combine the various diagnostically determined prismatic corrections into a single prism prescription. Applicants are not aware of a critical study that would have evaluated how the efficacy and variability of prism prescriptions depended on the possible combinations of the determined prismatic corrections.
For all of the above reasons, determining the prismatic power that optimally compensates accommodative misalignments remains a pressing medical need.